The liver has an enormous capacity to regenerate, as demonstrated by the 2/3 partial hepatectomy model in rodents. A stem cell compartment within the liver is activated when mature hepatocytes are unable to perform their role in this process. The overarching question of this research proposal is;which systemic signals regulate stem cell mediated liver regeneration and what molecular mechanisms underlie this regulation? The proposed research will identify the role and mechanism of action for specific factors involved in the activation, trafficking, expansion and differentiation of liver stem cells. Previous experiments have implicated connective tissue growth factor (CTGF), Notch-1, Wnt1, Somatostatin (SST), insulin-like growth factor binding protein 3 (IGFBP3) as playing key-roles in the liver stem cell response to 2- acetylaminofluorene/partial hepatectomy (2AAF/PH) liver injury in rats. All of these factors have a relationship to TGFb. We will characterize these relationships in three specific aims. Specific aim 1 will test the hypothesis that the TGF2/CTGF axis mediates the synthesis of a fibronectin rich provisional extracellular matrix (ECM) by activated portal fibroblasts that is required for the expansion of the liver stem cell population following 2-AAF/PH in rats. The main goal of this aim is to determine the role of TGFb and CTGF in forming the appropriate extracellular microenvironment for liver stem cell proliferation and migration. Specific aim 2 will test the hypothesis that IGFBP3 and SST mediate the trafficking of liver stem cells within the liver, and that SST potentiates the up-regulation of CTGF during the oval cell response to 2AAF/PH in rats. The role of these factors in directing the migration of liver stem cells and the interaction of these pathways with TGF2 signaling will be elucidated. Specific aim 3 will test the hypothesis that Notch/Jagged and Wnt/Frizzled signaling play a required role in lineage selection during liver stem cell differentiation. Each of these pathways is influenced by TGFb, and the integration of these signals appears to determine the phenotype of the differentiated stem cell. The data generated by these studies will provide a more complete understanding of the regulation of liver stem cells. This knowledge will help to guide the development of strategies for the therapeutic use of stem cell transplants for the treatment of liver disease. PUBLIC HEALTH RELEVANCE: Only 5,000 livers suitable for transplant become available each year, while 18,000 patients await these organs. Clearly, an alternative to whole organ transplant is needed. The studies proposed within this application are designed to identify mechanisms that regulate repair of the liver by stem cells. These mechanisms may be targeted therapeutically to bolster stem cell transplants into diseased organs offering an alternative to whole organ transplant in many patients.